iio: st_sensors: add debugfs register read hook

This adds a debugfs hook to read/write registers in the ST
sensors using debugfs. Proved to be awesome help when trying
to debug why IRQs do not arrive.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/magnetometer/st_magn_core.c b/drivers/iio/magnetometer/st_magn_core.c
index f8dc4b8..b27f014 100644
--- a/drivers/iio/magnetometer/st_magn_core.c
+++ b/drivers/iio/magnetometer/st_magn_core.c
@@ -560,6 +560,7 @@
 	.attrs = &st_magn_attribute_group,
 	.read_raw = &st_magn_read_raw,
 	.write_raw = &st_magn_write_raw,
+	.debugfs_reg_access = &st_sensors_debugfs_reg_access,
 };
 
 #ifdef CONFIG_IIO_TRIGGER